Optical bus

ABSTRACT

An optical bus for distributing optical signals. In one form, the optical bus comprises an optical fiber comprising an integrated array of thermal switches at predetermined intervals. In another form, the optical bus comprises an optical fiber comprising an integrated array of optical beam splitters at predetermined intervals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical communications and, moreparticularly, to the coupling and distribution of optical signals.

2. Background of the Invention

In the area of optical communications, there is a need to distribute, orredirect, optical streams (gigabit per second rates or higher) from one,or more, source devices to one, or more, destination devices. Onesolution is to first convert an optical signal to its electricalequivalent and then apply the resulting electrical signal(s) to anelectrical bus, which is used to interconnect the various devices. (Asknown in the art, an electrical bus comprises one or more electricalconductors for distributing one or more electrical signals from one, ormore, source devices to one, or more, destination devices.)Unfortunately, there are added costs involved with this electricalconversion, e.g., there is the dollar cost of the electrical componentsthemselves that are required to convert the signal from an optical formto an electrical form and back again—and there is also a performancecost in terms of having to process high-speed gigabit optical signals inthe electrical domain that may introduce distortion and delay.

SUMMARY OF THE INVENTION

An optical bus for distributing optical signals. In particular, a lightguide comprises an array of embedded elements for coupling light betweena plurality of ports of the light guide.

In one embodiment, the light guide is an optical fiber comprising anintegrated array of thermal switches at predetermined intervals.

In another embodiment, the light guide is an optical fiber comprising anintegrated array of optical beam splitters at predetermined intervals.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an illustrative embodiment of an optical bus in accordancewith the principles of the invention;

FIGS. 2 and 3 show other views of the embodiment of FIG. 1;

FIG. 4 shows another embodiment of an optical bus in accordance with theprinciples of the invention;

FIG. 5 shows another view of the embodiment of FIG. 4;

FIG. 6 shows another embodiment of an optical bus in accordance with theprinciples of the invention; and

FIGS. 7 and 8 show illustrative applications of the inventive concept.

DETAILED DESCRIPTION

An illustrative embodiment of an optical bus in accordance with theprinciples of the invention is shown in FIG. 1. Optical Bus 100comprises a light guide with embedded elements for coupling lightbetween a plurality of ports of the light guide. Illustratively, thelight guide is represented by optical fiber 30 (presumed to be a singlemode optical fiber), and the embedded elements are represented by aplurality of thermal switches 35-1 through 35-N spaced at predetermined(e.g., uniform) intervals in optical fiber 30 (also referred to as an“in-line thermal optical bus”). Each thermal switch is controlled viathermal switch control element 40 (control signals 41-1 through 41-N).For reference purposes, lower numbered thermal switches are considered“upstream” of higher numbered thermal switches (or, higher numberedthermal switches are “downstream” of lower numbered thermal switches).For example, thermal switch 35-1 is upstream of thermal switch 35-3,while the latter is downstream of thermal switch 35-1. Other than theinventive concept, the elements shown in FIG. 1 are well-known and willnot be described in detail. For example, an optical fiber comprises acore and a cladding. Similarly, a thermal switch, by itself, is known inthe art, (e.g., a thermal switch can be purchased from Kymata Ltd. andinformation can be found at “www.kymata.com”). Likewise, thermal switchcontrol 40 represents a stored-program control based-processor (e.g., amicroprocessor) and associated memory (both not shown) for providingcontrol signals 41-1 through 41-N. It is presumed that thermal switchcontrol 40 is suitably programmed using conventional programmingtechniques, which, as such, will not be described herein.

Optical fiber 30 comprises N input ports (15-1 through 15-N) along itslength (physically, each input port of optical fiber 30 is an accesspoint, window, or hole, to let in light). Each thermal switch has twoinput ports and one output port (not explicitly shown in FIG. 1), i.e.,each thermal switch is a 1×2 switch. One input port of each thermalswitch is coupled through one of the N input ports of optical fiber 30to receive one of N input optical signals (from any of N optical sources(not shown)). The other input port of each thermal switch is coupled tothe core 31 of optical fiber 30. As can be observed from FIG. 1,effectively this other input port of each thermal switch is coupled tothe output port of the next downstream thermal switch. For example, oneinput port of thermal switch 35-1 is coupled to input port 15-1 forreceiving input optical signal 1, and the other input port of thermalswitch 35-1 is coupled (via that portion of core 31 of optical fiber 30)to the output port of thermal switch 35-2. The output port of thermalswitch 35-1 is coupled to that portion of core 31 of optical fiber 30that provides the output optical signal 36, via output port 25. (Itshould be noted that each input optical signal may comprise one or morewavelengths, e.g., an optical signal may have a single wavelength orhave multiple wavelengths (a WDM (wavelength division multiplexed)signal).)

Thermal switch control 40 controls the state of each thermal switch. Asused herein, when a thermal switch is “ON,” optical energy is coupledfrom that input port of the thermal switch that is coupled to acorresponding input port of optical fiber 30 to the output port of thethermal switch (and hence into core 31 of optical fiber 30). Conversely,when a thermal switch is “OFF,” optical energy is coupled from thatinput port of the thermal switch that is coupled to core 31 of opticalfiber 30 to the output port of the thermal switch (and hence back intocore 31 of optical fiber 30 for transmission further upstream).Consequently, when a thermal switch is “OFF,” light is propagatedthrough optical fiber 30, going upstream from one thermal switch to thenext upstream thermal switch. Conversely, when a thermal switch is “ON,”light is let into the core of the optical fiber and light from any otherdownstream thermal switch is blocked. For example, consider thermalswitch 35-3. When thermal switch 35-3 is controlled to be “ON,” lightapplied via input port 15-3 is propagated into core 31 of optical fiber30 via thermal switch 35-3. Assuming that thermal switches 35-1 and 35-2are “OFF,” this light is propagated through optical fiber 30 to becomeoutput optical signal 36 (albeit with some attenuation). Since thermalswitch 35-3 is “ON,” any downstream light propagating through opticalfiber 30, e.g., via downstream thermal switches 35-4 through 35-N, isblocked. Thus, in this illustrative embodiment, it is assumed that onlyone thermal switch is “ON” at a time to permit light from thecorresponding input port to enter optical fiber 30 for propagationthrough optical fiber 30 (and any upstream thermal switches) to emergeas the output optical signal 36. It should be noted that to compensatefor any attenuation loss of the input optical signal, an opticalamplifier (not shown in FIG. 1) may be used to amplify output opticalsignal 36. However, it is known that a thermal switch has a low amountof attenuation loss, which may relegate the use of an optical amplifierto larger optical bus structures.

Another view of the embodiment of FIG. 1 is shown in FIG. 2. In thelatter, optical fiber 30 is shown receiving N input optical signalsalong the length of optical fiber 30 (where each of the corresponding Ninput ports are located). Embedded thermal switches 35-1 through 35-Nare represented by slash marks “/.” Illustratively, thermal switch 35-3is “ON,” which (as described above) lets input optical signal 3 intooptical fiber 30, which subsequently emerges as output optical signal36.

Similarly, another view of the embodiment of FIG. 1 is shown in FIG. 3.Here, a portion of optical fiber 30 is shown comprising the first threethermal switches, 35-1, 35-2 and 35-3. Each of these thermal switches iscoupled to a corresponding input port as illustrated by input ports15-1, 15-2 and 15-3. Illustratively, thermal switch 35-3 is “ON” andthermal switches 35-1 and 35-2 are “OFF,” which (as described above)lets input optical signal 3 into optical fiber 30, which subsequentlyemerges as output optical signal 36. In other words, optical fiber 30represents a light guide comprising an optical channel (core 31 ofoptical fiber 30) for conveying light such that a plurality of elementsare embedded along the optical channel, each embedded element disposedwithin the light guide for receiving light either from an associatedaccess point of the light guide (e.g., 15-1, 15-2, 15-3, etc.) or fromthe optical channel (e.g., core 31 of optical fiber 30).

An illustrative manufacturing procedure for a thermal optical bus asillustrated in FIGS. 1-3 is to use industry standard Silica-on-Silicontechniques (or other appropriate technology) and assemble embeddedthermal-optical 1×2 switches into an in-line array within a light guidestructure. (Silica glass is compatible with single mode fibers.) Thethermal switch array utilizes multiple thermal optical switches placedin-line with an associated input access point for receiving inputoptical signals along the length of the light guide structure. Theindividual thermal optical switches are controlled through individualheater control signals associated with each input signal position.

As noted above, when a thermal switch is “ON,” downstream light isblocked. This allows an input optical signal having one or morewavelengths (e.g., a WDM signal) to be propagated to the output port ofthe optical bus without mixing with other input optical signals.However, since the downstream light is blocked, an optical buscomprising integrated thermal switches is not conducive to generating aWDM signal from different input optical signals, each having a differentwavelength.

In contrast, another illustrative embodiment of an optical bus inaccordance with the principles of the invention is shown in FIG. 4.Optical Bus 200 comprises a light guide with embedded elements forcoupling light between a plurality of ports of the light guide.Illustratively, the light guide is represented by optical fiber 230(presumed to be a single mode optical fiber), and the embedded elementsare represented by a plurality of optical beam splitters (splitters)235-1 through 235-N spaced at predetermined (e.g., uniform) intervals inoptical fiber 230 (also referred to as an “in-line splitter opticalbus”). For reference purposes, lower numbered splitters are considered“upstream” of higher numbered splitters (or, higher -numbered splittersare “downstream” of lower numbered splitters). For example, splitter235-1 is upstream of splitter 235-3, while the latter is downstream ofsplitter 235-1. Other than the inventive concept, the elements shown inFIG. 4 are well-known and will not be described in detail. For example,an optical fiber comprises a core and a cladding. Similarly, a splitter,by itself, is known in the art.

Optical fiber 230 comprises N input ports (215-1 through 215-N) alongits length (again, physically, each input port of optical fiber 230 isan access point, window, or hole, to let in light). Each splitter hastwo input ports and one output port (not explicitly shown in FIG. 4).One input port of each splitter is coupled through one of the N inputports of optical fiber 230 to receive one of N input optical signals(from any of N optical sources (not shown)). The other input port ofeach splitter is coupled to core 231 of optical fiber 230. As can beobserved from FIG. 4, effectively this other input port of each splitteris coupled to the output port of the next downstream splitter. Forexample, one input port of splitter 235-1 is coupled to input port 215-1for receiving input optical signal 1 (via selector 220 (describedbelow)), the other input port of splitter 235-1 is coupled (via thatportion of core 231 of optical fiber 230) to the output port of splitter235-2. The output port of splitter 235-1 is coupled to that portion ofcore 231 of optical fiber 230 that provides the output optical signal236, via output port 225.

Unlike the embodiment illustrated in FIG. 1, a splitter does not blocklight, i.e., some light is reflected and some light is passed through.Thus, with respect to optical bus 200 (ignoring for the moment selector220) light applied at any input port will mix with light applied atother input ports. As such, an optical bus comprising splitters isuseful in (WDM) applications, since light of different wavelengths canbe applied to different ones of the input ports 215-1 through 215-N withthe result that output optical signal 236 is a WDM signal. (It shouldalso be noted that, strictly speaking, the input ports of optical fiber230 also serve as output ports, since some light is reflected. In otherwords, they are bi-directional ports since some light from downstreamsplitters will appear on the upstream input ports. Indeed, this featureis taken advantage of in one of the optical applications describedfurther below.)

However, in the situation where the input optical signals all have thesame wavelength it is necessary to block light from certain ones of theinput ports to prevent inadvertent mixing of the input optical signalswithin core 231 of optical fiber 230. (Indeed, it may even beadvantageous to do this in a WDM application.) Thus, selector 220 isused to block the input optical signals. Selector 220 (control signalsnot shown) enables one input optical signal at a time to be applied tooptical fiber 230. Although not necessary to the inventive concept,selector 220 is, e.g., an array of thermal 1×2 switches, with one of theinput ports of each thermal switch unused. Alternatively, selector 220is an array of liquid crystal pixel elements such as described and shownin the co-pending, commonly assigned U.S. Patent application ofRanganath et al., entitled “An Optical CrossBar Switch,” applicationSer. No. 09/478,630, filed on Jan. 6, 2000, now abandoned. Controlcircuitry for either element is straightforward and is not describedherein. Similar to the description of optical bus 100, each element ofselector 220 can be referred to as having an “ON” state, i.e., opticalenergy is coupled to the corresponding input port of optical fiber 230,and an “OFF” state, i.e., light is blocked from the corresponding inputport of optical fiber 230. This is illustrated in FIG. 4, where selector220, element 3, is turned “ON” to allow light to enter optical fiber 230via input port 215-3. Other elements of selector 220 are “OFF.” As such,only light from input optical signal 3 transits optical fiber 230 tobecome output optical signal 236 (albeit with some attenuation). Thus,in a non-WDM application, it is assumed that only one element ofselector 220 is on at a time to permit light from the correspondinginput port to enter optical fiber 230 for propagation through opticalfiber 230 to emerge as the output optical signal 236. (It should benoted that selector 220 can be controlled so as to let more than oneinput optical signal into optical fiber 230, if so desired.)

Another view of the embodiment of FIG. 4 is shown in FIG. 5. In thelatter, optical fiber 230 is shown receiving any one of N input opticalsignals along the length of optical fiber 230 (where each of thecorresponding N input ports are located). Embedded splitters 235-1through 235-N are represented by slash marks “/.” Illustratively,selector 220 (as described above) lets input optical signal 3 passthrough into optical fiber 230 and blocks all other input opticalsignals. Input optical signal 3 subsequently emerges as output opticalsignal 236.

This embodiment of an optical bus utilizes embedded splitter elementsthat may have a significant amount of attenuation loss to an inputoptical signal. (For example, an optical beam splitter may have a 60/40ratio, i.e., 60% of the light is let through, while 40% of the light isreflected.). As such, an optical amplifier may be used to amplify theoutput optical signal in order to compensate for loss through thesplitters. This is illustrated in FIG. 6, which shows output opticalsignal 236 being applied to optical amplifier 250, which provides anamplified optical signal 251. (This is in contrast to theabove-described optical bus 100, which utilizes thermal switch elementsthat have lower forward light loss.)

An illustrative manufacturing procedure for a beam splitter optical busas illustrated in FIGS. 4-6 is to use industry standardSilica-on-Silicon techniques (or other appropriate technology) andassemble embedded optical beam splitter elements into an in-line arraywithin a light guide structure. (Silica glass is compatible with singlemode fibers.) The optical beam splitter array utilizes multiple opticalbeam splitters placed in-line with an associated input access point forreceiving input optical signals along the length of the light guidestructure. The individual optical beam splitters are passive and requireno external control signals. The amount of coating on each optical beamsplitter determines the ratio of reflected light to transmitted light,which is predetermined by design. Standard methods of fusion splicing,photolithography and reactive ion etching may be used to construct theoptical bus.

Some illustrative applications of an optical bus in accordance with theprinciples of the invention are shown in FIGS. 7 and 8. FIG. 7illustrates the use of an optical bus as part of a programmable linebuild-out attenuator, which is used to receive an input optical signalof varying energy level (a “hot signal”) and to provide an outputoptical signal at a predefined energy level. In particular, theprogrammable line build-out attenuator comprises optical bus 330,optical bus 340, selector 320, tap 335 and gain control circuit 305,which further comprises photo detector 315 and analog-to-digital (A/D)element 310. An input optical signal is applied to optical bus 340. Thelatter is illustratively an inline splitter optical bus (as shown inFIG. 4). As noted above, each splitter both reflects and transmitslight. As such, as light passes through each splitter of optical bus340, varying degrees of attenuation are introduced into the inputoptical signal. Since each splitter both reflects and transmits light,some light appears as an output signal at, what heretofore were referredto as the input ports of the optical bus. These are shown in FIG. 7 asattenuation taps 0 dB (decibels), −5 dB, −10 dB and −20 dB. Each of theoutput signals from the attenuation taps is applied to an element ofselector 320. The latter is controlled by gain control circuit 305 viacontrol lines 311, 312, 313 and 314. In this example, it is assumed thatgain control circuit 305 turns “ON” element 3, via control line 313, ofselector 320 to enable light to pass through to a corresponding inputport of optical bus 330, which is also illustratively an inline splitteroptical bus. (However, it could also be an inline thermal optical bus asillustrated in FIG. 1. In this case, selector 320 is not needed and gaincontrol circuit 305 controls each thermal switch via control signals311, 312, 313 and 314.) Optical bus 330 provides an optical signal(again at a certain attenuation level) to tap 335, which provides theoutput optical signal and also provides an optical feedback signal togain control circuit 305. The latter receives the optical feedbacksignal via photo detector 315, which converts the optical signal into anelectrical signal that is provided to A/D converter element 310.Depending on the level of the optical feedback signal, A/D converterelement 310 operates such that a different one of the control signals311, 312, 313 and 314 is enabled to turn on a particular element ofselector 320. Thus, the energy level of the output optical signal iscontrolled by selecting different ones of the attenuation taps ofoptical bus 340.

Turning now to FIG. 8, an optical tilt control application of an opticalbus in accordance with the principles of the invention is shown. Theoptical tilt control comprises selectors 405, 410, and 415, and opticalbuses 440, 445, 450 and 430. It is assumed for this application that alloptical buses are of the inline splitter optical bus. This particularapplication takes advantage of the WDM capability of an inline splitteroptical bus. Optical buses 440, 445 and 450, each receive an inputoptical signal at a different wavelength as provided by correspondingselectors 405, 410 and 415 (the control signals for these selectors arenot shown). In particular, an input signal having a wavelength R isprovided to optical bus 440, an input signal having a wavelength G isprovided to optical bus 445 and an input signal having wavelength B isprovided to optical bus 450. Each optical bus provides their inputsignal (in attenuated form) to a corresponding input port of optical bus430. Optical bus 430 mixes each received signal to provide the flat tiltoutput signal, which is a WDM signal.

As described above, and in accordance with the invention, an optical busprovides significant advantages over mechanical or electrical busconstruction, resulting in low power, no moving parts (robustness), zerocross talk and low loss. In addition, the optical nature of this designallows any Ethernet framing to be preserved and thus preserves Qualityof Service (QoS), VLAN (virtual local area network) tagging and payload.Due to the optical design of the bus, standardized link aggregationprotocols may be applied as separate gigabit links. Applications formetro-ring, metro access and enterprise access is possible with this busdesign, and at a reduced cost.

The foregoing merely illustrates the principles of the invention and itwill thus be appreciated that those skilled in the art will be able todevise numerous alternative arrangements which, although not explicitlydescribed herein, embody the principles of the invention and are withinits spirit and scope. For example, although a light guide wasillustrated in the context of an optical fiber, the light guide couldalso be formed using optical integrated circuit techniques. Similarly,although the illustrative embodiment described the use of embeddedelements such as thermal switches and optical beam splitters, otherelements may be used such as “optical bubble-jet technology” elementsformed into an in-line array within a light guide structure. Such anarray utilizes multiple optical bubble elements placed in-line with anassociated input window for receiving input optical signals along thelength of the light guide structure. The individual optical bubbleelements are driven through individual control signals associated witheach individual input signal position.

What is claimed is:
 1. An optical bus, comprising: a light guidecomprising an optical channel for propagating an optical signal; aplurality of elements embedded along said optical channel, each of saidelements comprising two input ports for receiving the optical signalfrom said optical channel and for receiving a second optical signal froma respective associated access port in said light guide, wherein each ofsaid elements is controllable to propagate one of the two input opticalsignals along said optical channel and block the other of the two inputoptical signals from further propagation.
 2. The optical bus of claim 1,wherein said light guide is an optical fiber.
 3. The optical bus ofclaim 1, wherein the plurality of elements are thermal switches atpredetermined intervals in said optical channel.
 4. The optical bus ofclaim 3, further comprising a control element for providing controlsignals for each of the thermal switches.